1. Field of the Invention
The present invention concerns a method for measurement of the three-dimensional density distribution in bones as well as a C-arm x-ray apparatus for such a purpose.
2. Description of the Prior Art
An apparatus for measurement of the mineral content in bones is known from DE 696 20 869 T2 wherein the bones to be examined are successively irradiated with x-ray radiation of different energies. The mineral content in the bones is determined from the measured absorption of the x-ray radiation and using previously acquired reference values acquired by means of a reference subject. As a result, a two-dimensional density distribution is thereby obtained in g/cm2. Under the circumstances, an emerging (i.e., just beginning) variation of the bone density in the bones, in particular a decrease of the mineral content in a predetermined volume element cannot be detected with this known method. For diagnosis of osteoporosis, however, it is extremely important to be able to detect slight changes of the three-dimensional density distribution in the bones at the earliest point in time.
A number of methods for two-dimensional measurement of the bone density are known from the prospect “LEXXOS Digital Flash Beam Technology—Bidimensional Densitomer” published in 2003 by Diagnostic Medical Systems. A two-dimensional digital x-ray image is thereby generated by a linear scan using a single or pencil beam and a linear or punctiform detector. In addition to this, it is also possible to generate a two-dimensional digital x-ray image with a two-dimensionally-expanded x-ray beam and a two-dimensional detector system. A two-dimensional exposure of the entire body can be produced by a movement of the x-ray system relative to the patient bed in the x-direction and y-direction. It is therewith possible to two-dimensionally determine the bone density at selected locations of the acquired area.
For determination of the bone density, from the prospect “UPIS 5000”, published in 2003 by Diagnostic Medical Systems, it is also known to measure the bone density by means of ultrasound. Yet a variation of the bone density in a predetermined volume element still cannot be exactly detected.
Determination of the bone density by means of quantitative computed tomography is known from www.syngo.com/deutsch/ct-osteo-htm”. A number of slices of a subject, for example of a vertebral body, are thereby acquired. A bone phantom is measured with the acquisition at the same time. As a consequence of this, the quantitative values can be calibrated absolutely (see W. A. Kalender et al., “The European Spine Phantom—a tool for standardization and quality control in spinal bone mineral measurements by DXA and QCT”, Eur. J. Radiol., 1995, 20, 2, 83–92). The bone density at specific locations of the spinal column thus can be quantitatively determined.
Furthermore, a mobile x-ray apparatus is known, distributed by the company Siemens AG under the designation “SIREMOBIL Iso-C3D”. An x-ray source and a detector for acquisition of two-dimensional x-ray images are provided in opposing arrangement on a C-shaped arc. The x-ray source can be rotated around an isocenter together with the detector. With this known x-ray apparatus, it is possible also to qualitatively, three-dimensionally show a subject to be examined.